Method for treating surface of magnesium-based metal to give metallic texture thereof

ABSTRACT

A method for treating a surface of a magnesium-based metal is disclosed for realizing a metallic texture of the magnesium-based metal. The method includes a buffing step of chemically polishing a surface of a magnesium-based metal by using a chemical polishing agent containing sodium nitride and sodium citrate; an immersing step of immersing the magnesium-based metal, which has been subject to the buffing step, into a strongly alkaline electrolyte solution of pH 11 or above; and an anodizing treatment step of forming a transparent anodizing film on the surface of the magnesium-based metal by applying current with a current density of 0.01 to 1 A/dm 2  to the magnesium-based metal in the strongly alkaline electrolyte solution.

BACKGROUND

1. Technical Field

The present disclosure relates to a method for treating a surface of a magnesium-based metal, and more particularly, to a method for treating a surface of a magnesium-based metal to give a metallic texture thereof with an environment-friendly scheme.

2. Description of the Related Art

A magnesium-based metal has been expected to be the most promising engineering material in the future, which can be substituted for an aluminum-based metal, in aspect of weight.

At present, the price of magnesium is substantially equal to that of aluminum, but magnesium is lighter than aluminum. As such, magnesium is more advantageous than aluminum for vehicles, airplanes, laptop computers, cellular phones, etc. in which light weight is more important. For example, in the field of vehicles, magnesium alloys have recently been used instead of steel or aluminum alloys in order to decrease the weight of vehicles for the purpose of improving the fuel efficiency.

In these days, even in aspect of environments, magnesium alloys having excellent recycling features tend to be widely used as structural metallic material. For example, existing plastic parts tend to be replaced with magnesium alloys with excellent recycling features, particularly for cases of notebooks, personal computers and cellular phones in the field of home appliances.

Such magnesium alloys have the most chemical activity among commonly used metals. Thus, if magnesium alloys are not subject to surface treatment, they are characterized to be very fast corroded under the atmosphere or in the solution, so it is important to form more dense and uniform coatings through the surface treatment process of magnesium alloys than the case of steel or aluminum alloys. However, it is extremely difficult to form a dense and uniform coating on magnesium alloys. This is because the surface of magnesium alloys is chemically irregular. Macro segregations and micro segregations cause the surface of magnesium alloys to be chemically irregular, and thus it is difficult to form a dense and uniform coating on magnesium alloys. In addition, since an oxide film generated on the surface of magnesium alloys is composed of Mg(OH)₂, which is an impermeable oxide film, a metallic texture inherent in magnesium may not be realized.

Among various surface treatment methods for magnesium alloys, a conversion treatment method or an anodizing treatment method has been most frequently used. Both methods are performed after any pre-treatment process such as a degreasing process and/or an acid pickling process. However, any functionality may be added to a surface of magnesium alloys only by using the anodizing treatment method.

Among the conventional surface treatment methods for magnesium alloys, a HAE method, a DOW17 method and a galvanic method have been widely known as the anodizing treatment method in the art, but there is a disadvantage in that these methods use an electrolyte solution containing heavy metals such as manganese and/or chrome, therefore causing waste water containing heavy metals to be generated and donating noxiousness to resultant products.

In addition, in the conventional anodizing treatment methods for magnesium alloys, an oxide film has been formed with a high voltage of 100V or above in a strongly alkaline electrolyte solution as disclosed in Korean Laid-open Patent Publication No. 10-2004-94105 or an impermeable oxide film has been formed by applying AC voltage of −200 to 400V in a pulse manner in a weakly alkaline electrolyte solution as disclosed in Korean Laid-open Patent Publication No. 10-2003-40824.

However, since the above conventional techniques result in a colored impermeable film having white or brown color, it is difficult to realize a metallic texture inherent in a magnesium-based metal. Thus, a technique has been sought in the art for realizing a metallic texture inherent in magnesium alloys by forming a transparent anodizing film on the surface of magnesium alloys. In addition, in order to form a qualified anodizing film on the surface of magnesium alloys, there is a need for a process for changing the surface of the magnesium alloys into another surface which is suitable for the formation of the anodizing film. As a polishing technique for the pre-treatment process of the anodizing treatment method, a chemical polishing technique using a chemical polishing agent containing chromic acid and HF has been well known in the art. However, this technique has been seriously criticized since the chemical polishing agent as described above is highly expensive and dangerous.

BRIEF SUMMARY

In one embodiment, a method is provided for treating a surface of a magnesium-based metal, in which a qualified transparent anodizing film for realizing a metallic texture of the magnesium-based metal may be formed on the surface of the magnesium-based metal, while using an environment-friendly electrolyte solution.

In one embodiment, a method is provided for treating a surface of a magnesium-based metal, in which a more qualified anodizing film may be realized by means of a cheaper and safer chemical polishing process before forming the anodizing film

In one embodiment, a method is provided for treating a surface of a magnesium-based metal, which includes a buffing step of chemically polishing the surface of the magnesium-based metal by using a chemical polishing agent containing sodium nitride and sodium citrate; an immersing step of immersing the magnesium-based metal, which has been subject to the buffing step, into a strongly alkaline electrolyte solution of pH 11 or above; and an anodizing treatment step of forming a transparent anodizing film on the surface of the magnesium-based metal by applying current with a current density of 0.01 to 1 A/dm² to the magnesium-based metal in the strongly alkaline electrolyte solution.

In the anodizing treatment step, the current preferably has a current density of 0.01 to 1 A/dm². If the current density exceeds 1 A/dm², an irregular oxide film may be generated on the surface of the magnesium-based metal, and such an irregular oxide film gives a difficulty in realizing metallic texture thereof. Also, if the current density is less than 0.01 A/dm², it is difficult to form a transparent anodizing film. More preferably, the current density is 0.2 to 0.7 A/dm². At this time, the anodizing treatment process is executed for about 3 minutes. In addition, in the anodizing treatment step, a voltage is preferably restricted to 10V or below.

In the buffing step, it is preferable that the chemical polishing agent comprises sulfuric acid, nitric acid, sodium nitride and sodium citrate.

The electrolyte solution preferably includes 100 to 300 parts by weight of potassium hydroxide, 0.5 to 50 parts by weight of KF (potassium fluoride), 5 to 50 parts by weight of NaSiO₄ and 0.1 to 0.5 parts by weight of Al, and the electrolyte solution is preferably maintained at a temperature of 20 to 70° C. The oxide film obtained after the anodizing treatment step is so transparent to realize the metallic texture of the magnesium alloy, which results in a substructure for the oxide film. Here, if a pigment is immersed into the oxide film, the magnesium-based metal may look as if a color is given to the surface of the magnesium-based metal having the metallic texture. The pigment may be at least one selected from the group consisting of cobalt acetate, potassium permanganate, ammonium sulfide, ferric sulfate, potassium ferricyanide, nickel sulfate, copper sulfate and stannous sulfate.

It is possible to change the surface of the magnesium-based metal into another surface which is suitable for anodizing by means of a chemical polishing process using a cheaper and safer chemical polishing agent. In addition, by optimally performing the anodizing treatment process on the resultant surface by using an environment-friendly electrolyte solution containing no heavy metals, it is possible to make a high-grade magnesium-based metal product having a transparent anodizing film on the surface thereof, thereby realizing the metallic texture of the magnesium alloy.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The above and other features and advantages of the embodiments will become apparent from the following description of a preferred embodiment given in conjunction with the accompanying drawings, in which:

FIG. 1 is a flowchart illustrating a conventional method for treating a surface of a magnesium-based metal;

FIG. 2 is a flowchart illustrating a method for treating a surface of a magnesium-based metal according to an embodiment;

FIG. 3 is a schematic view showing a surface treatment apparatus of the magnesium-based metal, employed in an embodiment;

FIG. 4 is a photograph showing a surface of the magnesium-based metal on which a transparent anodizing film is formed after a chemical polishing and a surface adjustment (or, alkali degreasing), which is taken by an optical microscope; and

FIG. 5 is a photograph showing a surface of the magnesium-based metal on which an anodizing film is not formed, which is taken by an optical microscope, for the comparison of metallic texture with the photograph of FIG. 4.

DETAILED DESCRIPTION

Hereinafter, an embodiment of a method for treating a surface of magnesium-based metal will be described in detail with reference to the accompanying drawings. Throughout the drawings, like reference numerals are used to designate like elements.

FIG. 1 is a flowchart illustrating a conventional method for treating a surface of a magnesium-based metal while FIG. 2 is a flowchart illustrating a method for treating a surface of a magnesium-based metal according to an embodiment.

As shown in FIG. 2, the surface treatment method according to this embodiment includes a buffing step 21, a degreasing or surface adjusting step 22, an anodizing treatment step 23 and a water washing step 24.

The buffing step 21 uses a chemical polishing scheme. After the buffing step 21 and the degreasing or surface adjusting step 22 are performed, the magnesium-based metal is immersed in a strongly alkaline electrolyte solution for an anodizing treatment, and then subject to the anodizing treatment step 23 in the electrolyte solution. A magnesium-based metal product on which a transparent anodizing film is formed by means of the anodizing treatment step is then subject to any post-treatment step including the water washing step 24.

Hereinafter, only the buffing step 21 and the anodizing treatment step 23 among the above steps will be explained in more detail.

Buffing Step

A chemical polishing agent containing 100 g/L of sulfuric acid, 15 g/L of nitric acid, 130 g/L of sodium nitride and 150 g/L of sodium citrate was prepared. Magnesium alloy was immersed in the chemical polishing agent, prepared at the room temperature, for 5 to 30 seconds to chemically polish the surface of the magnesium alloy. The composition ratio of the chemical polishing agent is not limited to the above description, and the polishing process was found to be excellent when the component ratios of the chemical polishing agent are within the following ranges: 50 to 500 parts by weight of sulfuric acid, 10 to 100 parts by weight of nitric acid, 100 to 300 parts by weight of sodium nitride, and 100 to 300 parts by weight of sodium citrate.

After the chemical polishing process, an optical microscope was used to observe and/or check the metallic texture of the magnesium alloy, and it was also found that many holes are created in the surface. These holes contribute to enhancing an adhesive force with a transparent anodizing film, which will be formed by its following anodizing treatment step.

Before the buffing step, the alkali degreasing step may be conducted. In the alkali degreasing step, the magnesium alloy, which had not been subject to the buffing step, was immersed for about 5 minutes in a degreasing solution of 80° C., which was obtained by putting sodium hydroxide and sodium carbonate at a ratio of 7:1 into a distilled water.

After the buffing step, the surface adjusting step may be conducted. In the surface adjusting step, the magnesium alloy, which has been subject to the buffing step, was immersed for about 15 seconds in a surface adjusting solution, which contains 100 g/L of chromic acid, 20 g/L of iron nitride and 1 g/L of fluoric acid. At this time, it is desirable that a stirring is accompanied in the surface adjusting step.

Anodizing Treatment Step

The anodizing treatment step is conducted in a strongly alkaline electrolyte solution. Process conditions capable of giving an effect on coating characteristics are composition of the electrolyte solution, current density, temperature, working time and so on. Among these process conditions, the composition of the electrolyte solution and the current density are most important. A strongly alkaline electrolyte solution employed in this embodiment should include 50 to 300 g of potassium hydroxide per 1 liter of the entire solution in order to ensure the strong alkali of pH 11 or above. Since potassium hydroxide is not a heavy metal, it may be environment-friendly. In more detail, the electrolyte solution to be used in the embodiment includes 100 to 300 g of potassium hydroxide per 1 liter of the entire solution, 0.5 to 50 g/L of KF, 5 to 50 g/L of NaSiO₄ and 0.1 to 0.5 g/L of Al.

If magnesium or magnesium alloy is immersed in the strongly alkaline electrolyte solution of pH 11 or above prepared as described above, an oxide film is well formed at a voltage of 10V or less.

Meanwhile, if a pigment is added to the electrolyte solution, various texture effects may be obtained.

The pigment to be added may be varied depending on a desired color. If at least one of cobalt acetate, potassium permanganate, ammonium sulfide, ferric sulfate, potassium ferricyanide, nickel sulfate, copper sulfate and stannous sulfate is added, various colors such as red, orange, yellow, bluish green, blue and black may be given to the oxide film.

In addition, if such an anodizing treatment process is conducted in order to realize the metallic texture, any possible problem relating to working environments, which might occur in a separate painting process, may be fundamentally prevented.

As shown in FIG. 3, after a magnesium or magnesium alloy 3 and a cathode substrate 4 are immersed in an electrolyte solution 2 received in an electrolytic bath 1, an anode of a rectified power supply 5 is connected to the magnesium alloy 3, and a cathode of the rectified power supply 5 is connected to the cathode substrate 4, so that a voltage is applied therebetween, thereby allowing a transparent anodizing film to be formed.

At this time, a current density was adjusted to 0.01 to 1 A/dm², more preferably 0.2 to 0.7 A/dm², and a voltage was restricted to 10V or below. As such, a uniform and dense film was formed on a surface of the magnesium or magnesium alloy 3. The electrolyte solution is maintained at a temperature of 20 to 70° C.

In this embodiment, the surface adjustment should be executed when the surface of the magnesium-based metal is to be adjusted. In other words, the surface adjustment is generally executed for the purpose of adjusting the surface of the magnesium-based metal after the buffing process is conducted. However, the surface adjustment should be properly executed depending on the performance desired for the surface treatment or the contamination level of the surface to be treated.

FIG. 4 is a photograph showing fine tissues in the surface of the magnesium alloy on which an anodizing film is formed by means of the anodizing treatment according to an embodiment, and FIG. 5 is a photograph showing fine tissues in the surface of the magnesium alloy itself before the surface treatment is conducted.

Comparing FIG. 4 with FIG. 5, although an anodizing film is formed on the surface shown in FIG. 4, the surface shown in FIG. 4 exhibits substantially the same metallic texture as the surface of the magnesium alloy shown in FIG. 5 on which an anodizing film is not formed. Only widths between two lines are more narrowed. This is because the refraction of light is varied due to the formation of the transparent anodizing film.

The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if desired to employ concepts of the various patents, applications and publications to provide yet further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure. 

1. A method for treating a surface of a magnesium-based metal, comprising: buffing the surface of the magnesium-based metal by chemically polishing the surface of the magnesium-based metal; immersing the magnesium-based metal, which has been subjected to the buffing, into a strongly alkaline electrolyte solution of pH 11 or above; and performing an anodizing treatment including forming a transparent anodizing film on the surface of the magnesium-based metal by applying current with a current density of 0.01 to 1 A/dm² to the magnesium-based metal in the strongly alkaline electrolyte solution.
 2. The method of claim 1, wherein the chemically polishing includes chemically polishing the surface of the magnesium-based metal with a chemical polishing agent containing sodium nitride and sodium citrate.
 3. The method of claim 2, wherein the chemical polishing agent comprises sulfuric acid, nitric acid, sodium nitride and sodium citrate.
 4. The method of claim 1, wherein, a voltage is restricted to 10V or below in the anodizing treatment.
 5. The method of claim 1, wherein in the anodizing treatment, the electrolyte solution includes 100 to 300 parts by weight of potassium hydroxide, 0.5 to 50 parts by weight of KF, 5 to 50 parts by weight of NaSiO₄ and 0.1 to 0.5 parts by weight of Al, and the electrolyte solution is maintained at a temperature of 20 to 70° C.
 6. The method of claim 1, wherein in the anodizing treatment, at least one of cobalt acetate, potassium permanganate, ammonium sulfide, ferric sulfate, potassium ferricyanide, nickel sulfate, copper sulfate and stannous sulfate is added to the electrolyte solution.
 7. A magnesium-based metal having a surface treated according to a process comprising: buffing the surface of the magnesium-based metal by chemically polishing the surface of the magnesium-based metal; immersing the magnesium-based metal, which has been subjected to the buffing, into a strongly alkaline electrolyte solution of pH 11 or above; and performing an anodizing treatment including forming a transparent anodizing film on the surface of the magnesium-based metal by applying current with a current density of 0.01 to 1 A/dm² to the magnesium-based metal in the strongly alkaline electrolyte solution.
 8. The magnesium-based metal of claim 7, wherein the chemically polishing includes chemically polishing the surface of the magnesium-based metal with a chemical polishing agent containing sodium nitride and sodium citrate.
 9. The magnesium-based metal of claim 8, wherein the chemical polishing agent comprises sulfuric acid, nitric acid, sodium nitride and sodium citrate.
 10. The magnesium-based metal of claim 7, wherein, a voltage is restricted to 10V or below in the anodizing treatment.
 11. The magnesium-based metal of claim 7, wherein in the anodizing treatment, the electrolyte solution includes 100 to 300 parts by weight of potassium hydroxide, 0.5 to 50 parts by weight of KF, 5 to 50 parts by weight of NaSiO₄ and 0.1 to 0.5 parts by weight of Al, and the electrolyte solution is maintained at a temperature of 20 to 70° C.
 12. The magnesium-based metal of claim 7, wherein in the anodizing treatment, at least one of cobalt acetate, potassium permanganate, ammonium sulfide, ferric sulfate, potassium ferricyanide, nickel sulfate, copper sulfate and stannous sulfate is added to the electrolyte solution. 